Reflectors fall into two generic classifications: diverging and converging. A diverging reflector scatters light directed toward it, and has a focal center generally on or behind the reflector surface, often being a convex or flat reflector rather than concave. Converging reflectors are in the family of concave reflectors having focal centers in front of the reflector surface. The extreme concave reflector is a circular reflector having a focal center at the radius. In that case, a lamp placed at the focal center directs light to the reflector, the reflector then reflecting the light back to the focal center. Less extreme converging reflectors are produced by elliptical surfaces wherein the two centers of the ellipse generate plural focal points. In the case of elliptical reflectors, light generated at one focal center is reflected to and converges at the second focal point.
Situated between the converging and diverging reflector classes is a reflector that generates a light pattern parallel to the reflector axis. The reflector is essentially a converging reflector, the converging point being located at infinity. This type of reflector is created by a parabolic surface, wherein the light source is placed at the first focal center, which is in fact the geometric center of the parabolic curve, directing light to the reflector surface and therefrom toward a second focal point at infinity. Thus, the light is actually parallel to the reflector axis defined by a line passing through the two focal centers of the reflector and the reflector zenith.
Therefore, a converging reflector is defined as any reflector having a configuration wherein the two sides of the reflector approach parallel asymptotes or actually converge at a fixed distant point, ranging anywhere from the parabola to the circle.
A diverging reflector may be created by simply distorting the parabola to a slight degree so that the light beam scatters rather than converges at infinity. Essentially, diverging means that the light converges at a point behind the reflector, i.e. the focal center behind or on the reflector surface. As the reflector surface becomes less concave, the more scattered the reflecting light beam. Thus, a diverging reflector includes straight, convex, and the family of concave configurations wider than a parabola.
Most lamp reflectors are necessarily confined to the family of converging reflectors, in order to provide a controlled beam pattern. To generate the necessary variable intensity for large surface lighting the reflector is generally a combination of two or more distinct converging reflector surfaces.
The focal centers of a reflector generally are the geometric centers of the curve defining the reflector cross-section, light being directed therefrom to the reflector surface and then reflecting from the reflector surface at a reflection angle equal to the angle of incidence. Thus, when an elliptical surface is used, the light emitted from one focal point automatically converges at the second focal point. Likewise, when a parabolic reflector is utilized and a light source is placed at the geometric center, the reflected light beam is parallel to the reflector axis, the light beam theoretically converging at a second focal point at infinity.
The point on the reflector surface at which a normal line extending therefrom also passes through the geometric or focal center of the reflector curve is defined as the reflector zenith. This normal line is defined as the axis of the reflector.
The present invention relates to concave reflectors and luminaires for use in asymmetrical light distribution. When illuminating a vertical surface such as a building facia, an advertising sign or the like, it is necessary to provide a variable lighting intensity generating uniform coverge over the entire surface area, to provide visual appeal and readability to the illuminated surface.
The ideal lighting luminaire should have an optical system that will generate a controlled asymmetric vertical light beam pattern on a surface to be illuminated. Maximum candle power should occur at the upper limits of the working beam with a linear decrease in candle power toward the bottom of the beam. The lateral beam plane should have minimal candle power decrease throughout a wide horizontal sweep. This permits illumination of a vertical planar surface with substantially uniform brightness from top to bottom while retaining broadest uniform lateral coverage across the illuminated surface in those instances in which the luminaire is mounted at or adjacent the base of said surface.
In the past, two basic optical designs have been utilized for vertical surface lighting applications: asymmetrical reflector surface contours with lamps positioned either parallel or perpendicular to the surface, and symmetrical reflector surface contours with the lamps positioned parallel to the surface.
The asymmetric reflector contour with lamps positioned parallel to a planar surface has the capabilities to develop an asymmetric beam pattern having acceptable vertical illumination patterns for overall surface illumination. However, each reflector configuration produces only a single pattern of light, limiting application of the asymmetric reflector to a single purpose. The The asymmetric reflector contour with a lamp positioned perpendicular to a vertical surface provides generally undesirable asymmetric light patterns due to the length of the light source arc tube. When utilizing asymmetric reflector configurations, an asymmetric light pattern is realized in the horizontal as well as the vertical plane. The beam classification is wide angle, producing but one single beam.
Symmetric reflector contours with lamps positioned parallel to a vertical surface generate a controlled symmetric vertical and horizontal light pattern with a maximum beam width in the lateral or horizontal plane. The symmetric light pattern will generate the maximum candle power at the center of the beam, that is, along the axis of the reflector, with decreasing candle power as the distance increases from the axis. The light pattern is effective in the horizontal plane, however, the vertical portions of the beam will not effectively cover the surface height to achieve proper uniformity. Normally, the maximum candle power at the center of the beam is aimed at the top of the surface to be illuminated in an attempt to satisfy the uniformity requirement. This method will waste the upper half of the light beam, causing lower total lumen efficiency of the light generated by the luminaire.
The present invention utilizes both asymmetric and symmetric working beam patterns to produce an efficient, uniform light pattern on a vertical surface. The reflector of the present invention consists of two identical reflector halves that are coupled together to produce various predetermined light patterns. The reflector half-sections are defined by parabolic contours in single or multiple steps. Each reflector half-section is mechanically identical with the exception of the reflecting surface finish. The surface finishes range in various degrees of diffused to specular surfaces and account for the different asymmetric vertical light patterns when used in combination while retaining the symmetric horizontal patterns of a symmetric reflector. A proper selection of reflector half combinations, when angularly directed at the vertical surface, will result in higher total lumen efficiency and a controlled uniform brightness coverage on the lighted surface. The selection of proper reflector combinations will also permit mounting locations closer to a vertical surface than previously practical. The range of reflector combinations provides the lighting designer with proper optical selections, particularly where mounting locations are limited.
Prior art devices can only generate an asymmetrical vertical light pattern by utilizing asymmetric reflector halves, thus decreasing the symmetrical wide range lateral or horizontal pattern required when lighting a perpendicular planar surface. The present invention permits the retention of ideal symmetrical reflectors for maintaining a wide lateral intensity while permitting a varying beam strength in the vertical direction, thus permitting substantially even intensity on a planar illuminated surface. Further, by utilizing the proper selected surface finishes, the amount of wasted light can be greatly reduced, thereby contributing to the overall lamp efficiency and aesthetic appearance of the illuminated surface. The versatility of the present invention permits the utilization of the reflector sections to produce a variety of asymmetric or, where desired, symmetric lighting patterns. For example, the symmetric pattern for wide, narrow, or medium light patterns may be generated, as well as asymmetric patterns combining any combination of wide, medium, and narrow beam patterns. Thus, the present invention presents a basic and fundamental variance from prior art systems, in that it provides a reflector for generating an asymmetric light pattern with the use of any of a plurality of symmetric reflector configurations.
All other attempts to produce a suitable asymmetric pattern to provide substantially uniform intensity on a vertical lighted surface have utilized asymmetric reflectors. An example of a suitable asymmetric reflector, providing a vertical lighting pattern is disclosed in U.S. Pat. No. 3,679,893. As illustrated therein, the reflector comprises two portions, one portion being an elliptical reflector section while the other portion consists of the plurality of parabolic reflector sections to produce the particular lighting pattern illustrated in FIGS. 4 and 5 of that patent. As can be seen, the candle power is greatest at the top of the beam and least at the bottom to provide substantially uniform illumination of a definite or particular light pattern. However, use of the asymmetric reflector of U.S. Pat. No. 3,679,893 will reduce the efficiency and desired output of the lateral or horizontal range of the lamp. If a different pattern of illumination is desired, that is, one having either less or greater height than illustrated in FIGS. 4 and 5 of the aforesaid patent, it becomes necessary to redesign the shape of the two reflective portions which collectively constitute the reflector of said patent.
In sharp contrast thereto, the present invention enables a plurality of different, distinctive, and predictable light patterns to be obtained from a pair of identically shaped, mirror-image reflector half-sections by controlling the reflectance factor, that is, the reflective surface finish, per se, of each half-section.
Further, the present invention is particularly versatile, generating an entire family of light patterns, reducing the amount of wasted light generated by the luminaire, thereby increasing the overall efficiency of the luminaire.